Field of the Invention
The present invention relates generally to threaded couplings, e.g., between two tubular members such as casing segments employed in the field of oil and gas recovery. In particular, the disclosure relates to the characterization and assessment of threaded couplings according to an anticipated fatigue performance of the threaded couplings subject to various cyclical loading conditions.
Brief Description of Related Art
A variety of wellbore operations include the connection of adjacent tubular members such as pipe, drill string, riser sections, casing segments, and the like. Some of the joints established between the tubular members include a threaded pin connector defined on one of the tubular members, which is engaged with a threaded box connector of another tubular member. Often, the threaded profiles forming the pin and box connectors are cut directly into the base metal of the tubular member. Thus, a cross-sectional area of the threaded profile is smaller than a cross-sectional area of a shank or pipe portion of the tubular member, and the connection is therefore structurally weaker than the pipe portion.
To assess the suitability of tubular members for a particular purpose, an operator often considers a stress amplification factor (SAF) of the connections between the tubular members. The SAF is used to account for the increase in the stresses caused by geometric stress amplifiers that occur within the tubular members, and is equal to the local peak stress in the tubular member divided by the nominal stress in the pipe wall section of the tubular member. Finite element analysis (FEA) techniques have been used to identify and quantify the local peak alternating stress in the connector under purely elastic loading conditions, and the local peak stress is then employed to calculate an SAF for the tubular members. A vendor will often offer this single value for the SAF for characterizing the tubular members, e.g., to help the user assess the relative suitability of design over another.
Equipment in the oilfield, however, is rarely loaded and operated in purely elastic conditions. Tubular members are routinely loaded to stress levels that induce plastic deformations in the connectors. Plastic deformations alter the geometry of the tubular members, and may alter the location and magnitude of the peak stress that the tubular members encounter during subsequent use. Since tubular members are regularly connected and disconnected many times, and reused over time, the initial FEA analysis and resulting SAF become less relevant to the expected performance of the tubular members over time. In light of the above, the inventors recognized the need for a methodology that provides a comprehensive analysis of the tubular members to assess the expected performance of the tubular members under realistic operating conditions.